An image forming apparatus for forming an image by the electrophotographic printing system includes such units as a photoreceptor, a charger, an exposing unit, and a developing unit. Because the properties of such units are changed in accordance with a change in environment and with time, the image forming condition of the image forming apparatus adopting the electrophotographic printing system is also changed in accordance with a change in environment such as temperature and humidity, and the total number of image formation. Thus, it is difficult to stably obtain a constant image forming condition.
In order to overcome this drawback, for the purpose of stabilizing the image by controlling image forming process conditions such as amount of charge, exposure amount, and development bias, a conventional image forming apparatus is provided with a variety of image stabilizing functions. Controlling of image forming process conditions is generally carried out at a predetermined timing when the power is turned on and when a predetermined number of copies have been made. For example, by changing the output of a static charger, a plurality of toner patch electrostatic latent images having different surface potentials are formed on the surface of a photoreceptor drum, and the toner patch electrostatic latent images are made visible into a plurality of toner patches having different toner density by supplying of developer from a developing unit, and the density of each toner patch made visible is detected by a photosensor. The charger output, which is an output of the static charger, which has produced a toner patch in the plurality of toner patches, whose detected density data coincide with reference data is used in the following copying processes.
Note that, since the number of toner patches formed is limited, it is not necessarily the case that the density of the toner patch in the plurality of toner patches coincides with the reference data. Thus, in the case where there is no detected density data coinciding with the reference data, detected density data P1 and P2 (P1&lt;P&lt;P2) close to the reference data are selected so as to determine the values of "a" and "b" in the following equations: EQU Vg1=aP1+b EQU Vg2=aP2+b
where Vg1 and Vg2 are charger outputs which have formed the toner patches of the detected density data P1 and P2, respectively. Then, the charger output Vg for obtaining the density of reference data P is determined using the following equation. EQU Vg=aP+b
Also, as another method for controlling the processing conditions based on the toner patch formed in the processing control, for example, Japanese Unexamined Patent Publication No. 51551/1994 (Tokukaihei 6-51551) discloses an arrangement wherein a toner patch is formed on a predetermined region on a photoreceptor, and image forming process conditions are corrected based on the difference in density of the toner patch and the non-image region.
Also, Japanese Unexamined Patent Publication No. 19259/1994 (Tokukaihei 6-19259) discloses an arrangement wherein in toner of the toner patch region formed on the photoreceptor, the amount of toner remaining on the photoreceptor after transferring process is detected, and the charge eliminating output is controlled in accordance with the transfer efficiency determined from the detected amount of toner.
Further, Japanese Unexamined Patent Publication No. 97665/1986 (Tokukaisho 61-97665) discloses an arrangement wherein a toner patch is formed on a photoreceptor before scanning the document from an image which has been positioned beforehand in a vicinity of a document table in a direction orthogonal to the moving direction of the photoreceptor (sheet transporting direction on the surface of photoreceptor), and the image forming process conditions are controlled individually with respect to plurality of positions orthogonal to the moving direction of the photoreceptor so as to uniformalize the density with respect to the entire image.
However, in the case where the document density is uneven in a direction orthogonal to the sheet transporting direction on the surface of the photoreceptor, the image density, the under fogging, the amount of toner consumed, and the contamination in the apparatus also become uneven, resulting in instable image quality and poor economy. Also, in the case where the number of image formation with respect to a sheet having a particular size is greater than that of a sheet having other sizes, large numbers of sheets contact with a certain range of the surface of the photoreceptor in a direction orthogonal to the sheet transporting direction, causing an uneven wear on the certain range of the photoreceptor. This results in differences in the charging ability and the sensitivity of the photoreceptor in a direction orthogonal to the sheet transporting direction on the surface of the photoreceptor, making the image density nonuniform. In this manner, under certain conditions, due to the fact that the surface of the photoreceptor is worn and deteriorated unevenly in a direction orthogonal to the sheet transporting direction, which has a large influence on image formation, uneven image forming conditions are generated to the degree which cannot be handled properly by the conventional image stabilizing methods, and as a result, an image with a nonuniform image quality is often generated.